[Show abstract][Hide abstract] ABSTRACT: The SecA2 proteins are a special class of transport-associated ATPases that are related to the SecA component of the general Sec system, and are found in an increasingly large number of Gram-positive bacterial species. The SecA2 substrates are typically linked to the cell wall, but may be lipid-linked, peptidoglycan-linked, or non-covalently associated S-layer proteins. These substrates can have a significant impact on virulence of pathogenic organisms, but may also aid colonization by commensals. The SecA2 orthologues range from being highly similar to their SecA paralogues, to being distinctly different in apparent structure and function. Two broad classes of SecA2 are evident. One transports multiple substrates, and may interact with the general Sec system, or with an as yet unidentified transmembrane channel. The second type transports a single substrate, and is a component of the accessory Sec system, which includes the SecY paralogue SecY2 along with the accessory Sec proteins Asp1-3. Recent studies indicate that the latter three proteins may have a unique role in coordinating post-translational modification of the substrate with transport by SecA2. Comparative functional and phylogenetic analyses suggest that each SecA2 may be uniquely adapted for a specific type of substrate. This article is part of a Special Issue entitled:Protein trafficking & Secretion.
Preview · Article · Oct 2013 · Biochimica et Biophysica Acta
[Show abstract][Hide abstract] ABSTRACT: The serine-rich repeat glycoproteins of Gram-positive bacteria comprise a large family of cell wall proteins. Streptococcus agalactiae (group B streptococcus, GBS) expresses either Srr1 or Srr2 on its surface, depending on the strain. Srr1 has recently been shown to bind fibrinogen, and this interaction contributes to the pathogenesis of GBS meningitis. Although strains expressing Srr2 appear to be hypervirulent, no ligand for this adhesin has been described. We now demonstrate that Srr2 also binds human fibrinogen and that this interaction promotes GBS attachment to endothelial cells. Recombinant Srr1 and Srr2 bound fibrinogen in vitro, with affinities of KD = 2.1 × 10−5 and 3.7 × 10−6
m, respectively, as measured by surface plasmon resonance spectroscopy. The binding site for Srr1 and Srr2 was localized to tandem repeats 6–8 of the fibrinogen Aα chain. The structures of both the Srr1 and Srr2 binding regions were determined and, in combination with mutagenesis studies, suggest that both Srr1 and Srr2 interact with a segment of these repeats via a “dock, lock, and latch” mechanism. Moreover, properties of the latch region may account for the increased affinity between Srr2 and fibrinogen. Together, these studies identify how greater affinity of Srr2 for fibrinogen may contribute to the increased virulence associated with Srr2-expressing strains.
No preview · Article · Oct 2013 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The accessory Sec system of Streptococcus gordonii is comprised of SecY2, SecA2, and five proteins (Asp1 through -5) that are required for the export of a serine-rich glycoprotein,
GspB. We have previously shown that a number of the Asps interact with GspB, SecA2, or each other. To further define the roles
of these Asps in export, we examined their subcellular localization in S. gordonii and in Escherichia coli expressing the streptococcal accessory Sec system. In particular, we assessed how the locations of these accessory Sec proteins
were altered by the presence of other components. Using fluorescence microscopy, we found in E. coli that SecA2 localized within multiple foci at the cell membrane, regardless of whether other accessory Sec proteins were expressed.
Asp2 alone localized to the cell poles but formed a similar punctate pattern at the membrane when SecA2 was present. Asp1
and Asp3 localized diffusely in the cytosol when expressed alone or with SecA2. However, these proteins redistributed to the
membrane in a punctate arrangement when all of the accessory Sec components were present. Cell fractionation studies with
S. gordonii further corroborated these microscopy results. Collectively, these findings indicate that Asp1 to -3 are not integral membrane
proteins that form structural parts of the translocation channel. Instead, SecA2 serves as a docking site for Asp2, which
in turn attracts a complex of Asp1 and Asp3 to the membrane. These protein interactions may be important for the trafficking
of GspB to the cell membrane and its subsequent translocation.
Preview · Article · Nov 2012 · Journal of bacteriology
[Show abstract][Hide abstract] ABSTRACT: The accessory Sec system is a specialized transport system that exports serine-rich repeat (SRR) glycoproteins of Gram-positive
bacteria. This system contains two homologues of the general secretory (Sec) pathway (SecA2 and SecY2) and several other essential
proteins (Asp1 to Asp5) that share no homology to proteins of known function. In Streptococcus gordonii, Asp2 is required for the transport of the SRR adhesin GspB, but its role in export is unknown. Tertiary structure predictions
suggest that the carboxyl terminus of Asp2 resembles the catalytic region of numerous enzymes that function through a Ser-Asp-His
catalytic triad. Sequence alignment of all Asp2 homologues identified a highly conserved pentapeptide motif (Gly-X-Ser362-X-Gly) typical of most Ser-Asp-His catalytic triads, where Ser forms the reactive residue. Site-directed mutagenesis of residues
comprising the predicted catalytic triad of Asp2 of S. gordonii had no effect upon GspB transport but did result in a marked change in the electrophoretic mobility of the protein. Lectin-binding
studies and monosaccharide content analysis of this altered glycoform revealed an increase in glucosamine deposition. Random
mutagenesis of the Asp2 region containing this catalytic domain also disrupted GspB transport. Collectively, our findings
suggest that Asp2 is a bifunctional protein that is essential for both GspB transport and correct glycosylation. The catalytic
domain may be responsible for controlling the glycosylation of GspB, while other surrounding regions are functionally required
for glycoprotein transport.
Preview · Article · Aug 2012 · Journal of bacteriology
[Show abstract][Hide abstract] ABSTRACT: The accessory Sec systems of streptococci and staphylococci mediate the transport of a family of large, serine-rich glycoproteins to the bacterial cell surface. These systems are comprised of SecA2, SecY2, and three core accessory Sec proteins (Asp1-3). In Streptococcus gordonii, transport of the serine-rich glycoprotein GspB requires both a unique 90-residue N-terminal signal peptide and an adjacent 24-residue segment (the AST domain). We used in vivo site-specific photo-cross-linking to identify proteins that interact with the AST domain during transport. To facilitate this analysis, the entire accessory Sec system of S. gordonii was expressed in Escherichia coli. The determinants of GspB trafficking to the accessory Sec system in E. coli matched those in S. gordonii, establishing the validity of this approach. When the photo-cross-linker was placed within the AST domain, the preprotein was found to cross-link to SecA2. Importantly, no cross-linking to SecA was detected. Cross-linking of the N-terminal end of the AST domain to SecA2 occurred regardless of whether Asp1-3 were present. However, cross-linking to the C-terminal end was dependent on the Asps. The combined results indicate that full engagement of the AST domain by SecA2 is modulated by one or more of the Asps, and suggest that this process is important for initiating transport.
Preview · Article · May 2012 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: GspB is a serine-rich glycoprotein adhesin of Streptococcus gordonii that is exported to the bacterial surface by the accessory Sec system. This dedicated export pathway is comprised of seven
components (SecA2, SecY2, and five accessory Sec proteins [Asp1 to Asp5]). The latter proteins have no known homologs beyond
the Asps of other species. Asp1 to Asp3 are absolutely required for export of the substrate GspB, but their roles in this
process are unknown. Using copurification analysis and far-Western blotting, we found that Asp2 and Asp3 could individually
bind the serine-rich repeat (SRR) domains of GspB. Deletion of both SRR regions of GspB led to a decrease in its export, suggesting
that binding of the Asps to the SRR regions is important for GspB transport by the accessory Sec system. The Asps also bound
a heterologous substrate for the accessory Sec system containing a slow-folding MalE variant, but they did not bind wild-type
MalE. The combined results indicate that the Asps may recognize the export substrate through preferential interactions with
its unstructured or unfolded regions. Glycosylation of the SRR domains on GspB prevented Asp binding, suggesting that binding
of the Asps to the preprotein occurs prior to its full glycosylation. Together, these findings suggest that Asp2 and Asp3
are likely to function in part as chaperones in the early phase of GspB transport.
Preview · Article · Jul 2011 · Journal of bacteriology
[Show abstract][Hide abstract] ABSTRACT: Bacterial binding to human platelets is an important step in the pathogenesis of infective endocarditis. Streptococcus gordonii can mediate its platelet attachment through a cell wall glycoprotein termed GspB ('gordonii surface protein B'). GspB export is mediated by a seven-component accessory Sec system, containing two homologues of the general secretory pathway (SecA2 and SecY2) and five accessory Sec proteins (Asps1-5). Here we show that the Asps are required for optimal export of GspB independent of the glycosylation process. Furthermore, yeast two-hybrid screening of the accessory Sec system revealed interactions occurring between Asp3 and the other components of the system. Asp3 was shown to bind SecA2, Asp1, Asp2 and itself. Mutagenesis of Asp3 identified N- and C-terminal regions that are essential for GspB transport, and conserved residues within the C-terminal domain mediated Asp3 binding to other accessory Sec components. The loss of binding by Asp3 also resulted in an impaired ability of S. gordonii to secrete GspB. These studies indicate that Asp3 is a central element mediating multiple interactions among accessory Sec components that are essential for GspB transport to the cell surface.
Preview · Article · Oct 2010 · Molecular Microbiology
[Show abstract][Hide abstract] ABSTRACT: The binding of bacteria to human platelets is a likely central mechanism in the pathogenesis of infective endocarditis. We have previously found that platelet binding by Streptococcus mitis SF100 is mediated by surface components encoded by a lysogenic bacteriophage, SM1. We now demonstrate that SM1-encoded lysin contributes to platelet binding via its direct interaction with fibrinogen. Far Western blotting of platelets revealed that fibrinogen was the major membrane-associated protein bound by lysin. Analysis of lysin binding with purified fibrinogen in vitro confirmed that these proteins could bind directly, and that this interaction was both saturable and inhibitable. Lysin bound both the Aalpha and Bbeta chains of fibrinogen, but not the gamma subunit. Binding of lysin to the Bbeta chain was further localized to a region within the fibrinogen D fragment. Disruption of the SF100 lysin gene resulted in an 83+/-3.1% reduction (mean +/- SD) in binding to immobilized fibrinogen by this mutant strain (PS1006). Preincubation of this isogenic mutant with purified lysin restored fibrinogen binding to wild type levels. When tested in a co-infection model of endocarditis, loss of lysin expression resulted in a significant reduction in virulence, as measured by achievable bacterial densities (CFU/g) within vegetations, kidneys, and spleens. These results indicate that bacteriophage-encoded lysin is a multifunctional protein, representing a new class of fibrinogen-binding proteins. Lysin appears to be cell wall-associated through its interaction with choline. Once on the bacterial surface, lysin can bind fibrinogen directly, which appears to be an important interaction for the pathogenesis of endocarditis.
[Show abstract][Hide abstract] ABSTRACT: Group B streptococcus (GBS) remains a major cause of morbidity and mortality among newborn children. The bacterium is a commensal
organism colonizing the rectum and the gastrointestinal and urogenital tracts of adults, but it can be transmitted to neonates
by an ascending infection of the maternal genital tract or during parturition. We previously reported that a transposon insertion
disrupting rpoE resulted in the decreased survival of the mutant in the neonatal rat sepsis model of GBS infection. rpoE encodes the δ protein, a subunit of RNA polymerase (RNAP) that has been characterized in Bacillus species. In this study, we confirm the association of the δ protein with purified GBS RNAP and show that it is expressed
in strains representing all nine serotypes. Flow cytometric analysis of a reporter strain containing a transcriptional fusion
of the rpoE promoter to gfp revealed that, in vitro, this gene is continuously expressed. Analysis of δ expression in the transposon mutant by quantitative
Western blotting revealed a 10-fold reduction in relative abundance (which was linked to the attenuation in virulence that
was observed for this mutant) compared to that for the wild-type strain. These data suggest that a minimum intracellular concentration
of δ is necessary for this organism to cause disease.
Preview · Article · Apr 2006 · Journal of Bacteriology
[Show abstract][Hide abstract] ABSTRACT: Group B streptococci (GBS) usually behave as commensal organisms that asymptomatically colonize the gastrointestinal and urogenital
tracts of adults. However, GBS are also pathogens and the leading bacterial cause of life-threatening invasive disease in
neonates. While the events leading to transmission and disease in neonates remain unclear, GBS carriage and level of colonization
in the mother have been shown to be significant risk factors associated with invasive infection. Surface antigens represent
ideal vaccine targets for eliciting antibodies that can act as opsonins and/or inhibit colonization and invasion. Using a
genetic screen for exported proteins in GBS, we identified a gene, designated lrrG, that encodes a novel LPXTG anchored surface antigen containing leucine-rich repeat (LRR) motifs found in bacterial invasins
and other members of the LRR protein family. Southern blotting showed that lrrG was present in all GBS strains tested, representing the nine serotypes, and revealed the presence of an lrrG homologue in Streptococcus pyogenes. Recombinant LrrG protein was shown in vitro to adhere to epithelial cells in a dose-dependent manner, suggesting that it
may function as an adhesion factor in GBS. More importantly, immunization with recombinant LrrG elicited a strong immunoglobulin
G response in CBA/ca mice and protected against lethal challenge with virulent GBS. The data presented in this report suggest
that this conserved protein is a highly promising candidate antigen for use in a GBS vaccine.
Full-text · Article · Apr 2005 · Infection and Immunity